White light can be composed of coloured components using the principle of colour mixing, which relies on three colour-mixing equations. The colour mixing principle implies that for compositions containing only two coloured components, such as blue and yellow or red and blue-green, white light with a predetermined CCT can be obtained when the coloured components complement each other, i.e. both their chromaticity and RPRFs are exactly matched in a particular way. A set of three coloured components, such as red, green, and blue, can be used for composing white light with different CCTs and different colour rendition characteristics depending on the selection of the SPDs and RPRFs of each component. When four or more appropriate coloured components are employed, the three colour mixing equations yield no single solution for a predetermined chromaticity of white light, i.e. white light of the same chromaticity can be obtained within an infinite number of SPDs containing blends of coloured components with various RPRFs. This implies that for a particular set of four and more coloured primary sources, colour rendition characteristics of white light can be varied.
Tailoring the SPD of white light within a single lamp became very convenient with the development of solid-state lighting technology based on LEDs. LEDs are available with many colours, have small dimensions, and their principle of operation allows varying the output flux by driving current. Direct emission LEDs employ the principle of injection electroluminescence, which yields narrow-band emission with the spectral peak position controlled by varying the chemical contents and thickness of the light-generating (active) layers. Phosphor converted LEDs employ partial or complete conversion of electroluminescence to other wavelengths. The latter LEDs are used for the generation of both wide-band coloured emission and white light within a single package. In white phosphor conversion LEDs, the RPRFs of the coloured components are set by adjusting the concentration and size of the phosphor particles in the converter, the size and/or shape of the converter, and the distance of the converter from the electroluminescent chip. Another method of generating white is assembling LED packages with different chromaticity into clusters and using electronic circuits for the control of the partial fluxes of each group of emitters and using optical means for the uniform distribution of the colour-mixed emission. Both methods allow for the development of sources of light with predetermined or dynamically controlled colour rendition properties.
Such versatility in tailoring the colour rendition properties of illumination has been considered in numerous patents and publications of prior art. In the case of the clusters of coloured LEDs, tetrachormatic systems, which have a degree of freedom for tailoring colour rendition properties of white light within a particular set of primary emitters, have been widely considered. However, the proposed SPDs depend on the colour rendition metric used. For instance, D. A. Doughty et al. (U.S. Pat. No. 5,851,063, 1998) proposed a source of light composed of 4 groups of coloured LEDs with the wavelengths of the LEDs selected such that the general colour rendering index (Ra), which is defined by the International Commission of Illumination (Commission Internationale de l'Éclairage, CIE) using 8 test colour samples (CIE Publication No. 13.3, 1995), is at least approximately 80 or 85. H. F. Börner et al. (U.S. Pat. No. 6,234,645, 2001) disclosed a lighting system composed of four LEDs with the luminous efficacy and Ra having magnitudes in excess of predetermined values. In the subsequent journal publications, the trade-offs between LER and Ra, as well as the corresponding optimal wavelengths of LEDs for tetrachromatic and pentachromatic sources of light were established (A. Zukauskas et al., Appl. Phys. Lett., 80, 234, 2002). M. Shimizu et al. (U.S. Pat. No. 6,817,735, 2004 and U.S. Pat. No. 7,008,078, 2006) disclosed tetrachromatic solid-state sources of white light with Ra of at least 90 and with improved colour saturating ability, which was quantified as the gamut area of chromaticities of four CIE standard test colour samples. Later, polychromatic LED clusters have been optimized using an improved metric for the assessment of colour rendition properties of light sources, Colour Quality Scale (CQS; W. Davis and Y. Ohno, Proc. SPIE 5941, 59411G, 2005; W. Davis and Y. Ohno, Opt. Eng. 49, 033602, 2010), which relies on 15 test colour samples and an improved colour space and quantifies the colour rendition properties in terms of the general CQS, which does not penalize colour distortions due to increased chromatic saturation, and additional scales for colour fidelity, gamut area, and preference. W. W. Beer and G. R. Allen disclosed illumination systems composed of a plurality of organic and inorganic emitters and having an average increase of chroma for the 15 CQS test colour samples (U.S. Pat. No. 8,247,959, 2012).
The CRI metric with a small number of test colour samples was also used for the optimization of colour rendition properties of phosphor converted LEDs. For instance, trichromatic phosphor blends for white LEDs with the complete conversion of near-UV electroluminescence were disclosed by J. R. Sohn et al. (Ra>75; U.S. Pat. No. 7,332,855, 2008) and A. Nagatomi et al. (Ra>80; U.S. Pat. No. 7,345,418, 2008). For white LEDs with the partial conversion of blue electroluminescence, dichromatic phosphor blends that provide Ra>90 were disclosed by J. R. Sohn et al. (U.S. Pat. No. 7,911,127, 2011), H. Brunner et al. (U.S. Pat. No. 7,965,031, 2011), and K. N. Kim et al. (U.S. Pat. No. 8,017,961, 2011).
However, the above approaches to the optimization of solid-state sources of white light containing multiple coloured components are far from exploiting the advantages of solid-state lighting in the versatility of colour quality to a full extent. Most approaches rely on colour rendition metrics that use a small number of test colour samples (8-15) and assess either solely colour fidelity characteristics of white light (Ra) or integrate high fidelity and colour saturating ability (CQS). Moreover, the general CRI has been found to contradict visual ranking of solid-state sources of light (CIE Publication No. 177, 2007), whereas the CQS metric still suffers from a small number of test colour samples of very similar chroma and from the lack of psychophysical validation.
An advanced approach to assessing colour quality of light sources, which distinguishes between different colour rendition characteristics with high confidence, is based on analyzing colour shift vectors for any number of different test colour samples (A. Zukauskas et al., IEEE J. Sel. Top. Quantum Electron. 15, 1753, 2009; A. Zukauskas et al., J. Phys. D Appl. Phys. 43, 354006, 2010). The samples are computationally sorted to several groups depending on a type of the colour distortion that occurs when the reference source is replaced by that under assessment. The type of colour distortion is evaluated depending on the behaviour of the colour shift vector in respect of experimentally established just perceived differences of chromaticity and luminance. Then the relative numbers (percentages) of test colour samples that exhibit colour distortions of various types are defined as statistical colour quality indices: Colour Fidelity Index (CFI; percentage of the test colour samples having the colour shifts smaller than perceived chromaticity differences), Colour Saturation Index (CSI; percentage of the test colour samples having the colour shift vectors with a perceivable increase in chromatic saturation), Colour Dulling Index (CDI; percentage of the test colour samples having the colour shift vectors with a perceivable decrease in chromatic saturation), Hue Distortion Index (HDI; percentage of the test colour samples having the colour shift vectors with a perceivable distortion of hue), and Lightness Distortion Index (LDI; percentage of the test colour samples having the colour shift vectors with a perceivable distortion of lightness).
The statistical approach has been employed for the maximization of CFI of polychromatic white lamps composed of coloured LEDs (A. Zukauskas et al., U.S. Patent Appl. No 2009/0200907) as well as of white LEDs with both partial and complete conversion in phosphors (A. Zukauskas et al., U.S. Pat. No. 7,990,045, 2011 and A. Zukauskas et al., U.S. Patent Appl. No 2009/0231832, respectively). The same approach has been used for establishing the principle design rules for LED-based lamps with maximized CSI (A. Zukauskas et al., Opt. Express 18, 2287, 2010) and maximized CDI (A. Zukauskas et al., Opt. Express 20, 9755, 2012). A colour rendition engine, based on a tetrachromatic tunable SPD, which is a weighted sum of the SPD of a high-CSI (colour-saturating) trichromatic red-green-blue (RGB) LED cluster and the SPD of a high-CDI (colour-dulling) trichromatic amber-green-blue (AGB) LED cluster have been disclosed (A. Zukauskas et al., PCT Patent App. No WO2013009157, 2013). By varying the SPD weight parameter, the engine traverses all possible tetrachromatic (RAGB) blends, including that with the highest colour fidelity (maximal CFI). The results of the psychophysical assessment of the engine (A. Zukauskas et al., Opt. Express 20, 5356, 2012) have indicated that subjects show preferences to scenes illuminated by light that has both CSI and CFI high and of the same order (0.3≦CSI/CSI≦3) rather than to that with the highest CFI (subjectively identified as “most natural”). Such a quantification of preferential colour rendition properties of light allows for the development of solid-state sources of white light for preferential colour rendition having fixed SPDs and therefore a simplified control.
The prior art closest to the proposed solid-state sources of white light for preferential colour rendition is the aforementioned colour rendition engine disclosed in PCT Patent App. No WO2013009157, 2013. This engine requires simultaneous control of four groups coloured LEDs using a particular algorithm. Also, it features a multi-package design that can be considerably simplified by using single-package phosphor converted LEDs.